This invention relates to a method of etching successive substrates on a platen in a chamber in which the etching process results in carbonaceous deposits in the chamber.
A number of etching processes may result in carbonaceous deposits in the etched chamber. This is particularly true when processing polyimide (PI) coated silicon wafers, which are widely used in the semi-conductor and other related industries. Prior to metal deposition it is common and desirable to remove material from the wafer surface by a sputter etch process (typically using Ar) in order to ensure a high quality metal/metal interface. This step is normally conducted in a physical vapour deposition (PVD) pre-clean module. However, prolonged or frequent etching of PI wafers through the PVD pre-clean module can lead to a build-up of sputtered material around the chamber lid and walls. This carbonaceous material can accumulate to such a level that particles can become loose and begin to move around the chamber, particularly during processing, and there is the potential for them falling onto and contaminating the wafers.
It is known to seek to combat this problem by frequent use of an oxygen plasma. The oxygen step may be relatively short (e.g. 60 s) and act as a conditioning step where no platen bias is required. It is believed that the introduction of an oxygen plasma into a carbonised chamber will promote carbon oxidation, leading to CO/CO2 which may readily be pumped away. The removal of the material lowers the particle count.
As can be seen in FIG. 1, the Applicants have monitored the whole process through optical spectroscopy. During the initial stages of the oxygen plasma, O2 levels within the chamber are low whilst CO levels appear high. Presumably the Oxygen is converting the carbon species, present from the PI etching, to CO before reaching equilibrium.
The frequency of the oxygen plasma treatment tends to be selected by the customer depending on the rate of particle formation they experience and any other requirements they may have. However, for a typical Ar sputter clean etch where 150-300 Å SiO2 (or equivalent) is removed prior to the subsequent deposition step, it is likely that, within 50 to 100 PI coated wafers, an O2 plasma clean will be required. Although this process provides effective particle control, the Applicants have discovered that following such oxygen treatment there is a sharp reduction in etch rate directly afterwards. The drop in etch rate has been measured to be between 20 to 30%. This obviously leads to significant lack of uniformity between successive wafers and many semi-conductor manufacturers would not have accepted this initial drop in etch rate or the resultant drop in throughput, if they had been aware of the problem.